Particle system disasters if I edit the mesh
I'm assuming this happened with hair particles that were combed? Particle rely on faces and vertices, if you have 101 vertices and you delete no. 40, 41-100 have to change their index. Since the particles are linked to the indices, they go berserk.
The short answer: Don't do that. If you really need to remove geometry, use a mask modifier after the particles system.